The present invention is a method and a device for acquiring analog signals and converting the analog signals to digital signals with high precision.
Acquiring analog signals and converting them into digital signals is carried out in many fields. For example in the seismic prospecting field analog signals are picked up by geophones or hydrophones resulting from underground reflections which are recorded in order to establish a representation of the subsoil. These reflections are generally amplified, sampled and digitized in order to be available for various subsequent signal processing. Seismic signals have very wide dynamic variation because of the variation of the explored areas. These dynamic variations often exceed the digitizing range of the analog to digital converters used for signal processing which work best with 16 bits.
A method allowing enlargement of the dynamic range of analog signals which may be digitized through digital to analog converter of the type comprising an input for applying a stable reference voltage is described in French Patent 2 625 634. The prior art method comprises connecting to the input a voltage generator producing two voltages of the same magnitude but of opposite polarity. According to the polarity of the input signal to be digitized, either the positive reference voltage or the negative voltage is applied. This method provides enlargement of the dynamic range.
French Patent 2 626 423 describes another dynamic range enlarging system for an analog to digital converter comprising a fixed gain amplifier, a sampler and an analog-to-digital converter (ADC) to which a fixed reference voltage, selected from a set of stable voltages obtained through subdivisions of the same calibrated voltage, is applied. Selecting the reference voltage is achieved by a comparison between the sampled analog voltages and the stable voltages derived from the calibrated voltage. Such a system provides good preciseness in digitizing relatively weak input signals which considerably enlarges the conversion range dynamics if the input signals to be digitized are not too small with respect to the noise level of the analog to digital converter.
Binary-gain amplifiers, which are used for seismic acquisition, are typically designed for delivering amplified signals defined by a floating point mantissa which is lower than the maximum voltage digitizable by the A-D converter, and an exponent which is generally a power of the number two. The resulting numeric words comprise a specified number of bits defining the digitized value of the mantissa and a binary digitized value of the exponent. Binary-gain amplifiers provide good results if the gain of the different amplification stages producing the bits of the exponent of the amplified voltage are precisely known. This is not always the case. The gain of the stages, as far as is known with great precision at the time of manufacturing because of the adjustment difficulties, is subject to variations caused by temperature variations and time. Gain jumps may occur, which decrease the preciseness of the digitized signal samples. The total linearity of the amplifier followed by the converter is generally different from that of the converter alone.